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This final America's Climate Choices report is the conclusion of a five-report series that included:

Climate change is occurring, is very likely caused primarily by the emission of greenhouse gases from human activities, and poses significant risks for a range of human and natural systems. Emissions continue to increase, which will result in further change and greater risks. In the judgment of this report's authoring committee, the environmental, economic, and humanitarian risks posed by climate change indicate a pressing need for substantial action to limit the magnitude of climate change and to prepare for adapting to its impacts.

Although there is some uncertainty about future risk, there are many reasons why it is prudent to act now. The sooner that serious efforts to reduce greenhouse gas emissions proceed, the lower the risks posed by climate change, and the less pressure there will be to make larger, more rapid, and potentially more expensive reductions later. In addition, every day around the world, crucial investment decisions are made about equipment and infrastructure that can "lock in" commitments to greenhouse gas emissions for decades to come. Most actions taken to reduce vulnerability to climate change impacts are also common sense investments that will offer protection against natural climate variations and extreme events. Finally, while it may be possible to scale back or reverse many responses to climate change (if they somehow proved to be more stringent than actually needed), it is difficult or impossible to "undo" climate change, once manifested.

Current efforts of local, state, and private sector actors are important, but not likely to yield progress comparable to what could be achieved with the addition of strong federal policies that establish coherent national goals and incentives, and that promote strong U.S. engagement in international-level response efforts. The inherent complexities and uncertainties of climate change are best met by applying an iterative risk management framework and making efforts to: significantly reduce greenhouse gas emissions; prepare for adapting to impacts; invest in scientific research, technology development, and information systems; and facilitate engagement between scientific and technical experts and the many types of stakeholders making America's climate choices.

Key Messages

Climate change is occurring, is very likely caused primarily by human activities, and poses significant risks to humans and the environment. These risks indicate a pressing need for substantial action to limit the magnitude of climate change and to prepare for adapting to its impacts.

Decisions about the exact magnitude and speed of response efforts will depend on how much risk society deems acceptable. But in the Committee's judgment there are numerous motivations for action, including for instance:

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The faster that greenhouse gas emissions are reduced, the lower the risks; and the less pressure there is to make steeper and potentially more expensive reductions later.

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Investments currently being made in energy-related infrastructure and equipment will lock in emissions commitments for decades to come. Enacting relevant policies now will provide crucial guidance for those investment decisions.

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The risks of continuing "business as usual" are greater than the risks associated with strong efforts to limit and adapt to climate change. Policy changes can potentially be reversed or scaled back if needed, whereas many adverse changes in the climate system would be difficult or impossible to "undo".

Uncertainties in projecting future greenhouse gas emissions and in estimating the sensitivity of the climate system to greenhouse gases make it difficult to project the exact severity, location and timing of climate change impacts. Uncertainty is not a reason for inaction, however; it is, in fact, a compelling reason for action, especially given the possibilities of abrupt, unanticipated, and severe impacts.

Reducing greenhouse gas emissions would reduce the need for adaptation but not eliminate it. There is a need to begin mobilizing now to reduce vulnerability to climate change impacts.

Emissions reductions and adaptation efforts should be guided by an iterative risk management approach, which emphasizes taking action to reduce risks while continuously incorporating new information and adjusting efforts accordingly.

Response efforts currently being advanced by state and local governments, non-governmental organizations, and the private sector are significant, but not likely to yield progress comparable to what could be achieved with strong national policies and leadership.

A comprehensive U.S. response to climate change includes efforts to:

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Substantially reduce greenhouse gas emissions (ideally, through a national carbon pricing system and strategic complementary policies)

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Begin mobilizing for adaptation at all levels

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Invest in research and development, both to advance basic understanding and to improve/expand response options

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Develop effective systems to inform and evaluate climate choices

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Link scientific analysis with public deliberation

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Actively engage in international response efforts

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Coordinate the many related components of the nation's response efforts